This section provides background information related to the present disclosure which is not necessarily prior art.
Various industries use radio frequency (RF) energy to drive plasma chambers in order to fabricate various components such as integrated circuits, solar panels, compact disks (CDs), digital versatile (or video) discs (DVDs), and the like. Each fabrication process can vary depending upon the particular component being manufactured. The various processes often call for delivery of RF energy at varying frequencies, power levels, and efficiencies.
Traditionally, RF power delivery systems were each designed to satisfy requirements of a particular plasma manufacturing process. RF power amplifiers (PAs) and generators were thus not interchangeable or easily modified to accommodate various applications with different requirements. A variable class amplifier system (sometimes referred to as a Class O amplifier) has been introduced, which may be used to satisfy different classes of operation. The variable class amplifier system may be used to satisfy, for example, Class AB operation, Class E operation and classes between Class AB and Class E.
A variable class amplifier system may include, for example, two PA boards. Each of the PA boards may have a predetermined number of PAs (e.g., 4 PAs). Each PA has a respective capacitor-inductor-capacitor (CLC) network. The CLC networks are used to provide a selected class of operation. Each CLC network includes two capacitors and an inductor. As an example, a first capacitance of each of the CLC networks is connected between an output of a respective PA and a ground reference. The inductance of each of the CLC networks is connected between (i) the output of the respective PA and the first capacitance, and (ii) the second capacitance. The second capacitance is connected between the inductance and the ground reference.
The operating class of the variable class amplifier system may be adjusted by adjusting the capacitance and inductance values of each of the CLC networks. For example, the second capacitance values of the CLC networks may be increased for Class E operation. The second capacitance values of the CLC networks may be decreased for Class AB operation.
During class AB operation, transistor conduction angles of the PAs may be between 180-360°. Approximately half of the input wave cycle is amplified by a first switch, and the other half of the input wave cycle is amplified by a second switch operating in a complementary manner. Class AB operation is typically exemplified by each switch conducting a small amount during portions of cycles when the switches are generally OFF. This reduces dead zones, or periods when both switches are OFF, which minimizes or eliminates crossover. Class AB amplifiers typically have power efficiencies between 50-78.5%. In conventional PAs, class AB power efficiency can be limited to approximately 70%.
An ON state of a Class E amplifier occurs when voltage is at or near zero across a switch and when a high level of current is flowing through the switch. An OFF state of a Class E amplifier occurs when the voltage across the switch is at a high level and when current flowing through the switch is at or near zero. Thus, the switch performs as a low-resistance closed switch during an ON part of a RF cycle, and performs as an open switch during an OFF part of the RF cycle. Class E amplifiers can have efficiencies between 85-95%, but tend to exhibit a reduced amount of power output and/or linearity as compared to Class AB amplifiers.
Although the variable class amplifier system may be used to satisfy different classes of operation, the variable class amplifier system is susceptible to power output variations and current variations due to changes in input frequency. Certain PA applications experience wideband frequency operation due to automatic frequency tuning (AFT). Wideband frequency operation refers to an oscillation and/or change in an input frequency of a power amplifier system. The change in frequency may be, for example, ±5% from a center frequency. As an example, for a center frequency of 60 megahertz (MHz), an operating frequency range may be 57-63 MHz. This change in input frequency can result in impedance variations in transistors, transformers, and CLC networks of a power amplifier system. The impedances variations cause the variations in the RF output power and the PA power supply current with frequency, which negatively affects system operating efficiencies.